Metalloporphyrins have been used as ionophores in anion-selective electrodes for many years. They yield selectivity patterns that deviate from the Hofmeister sequence because of the selective axial ligation of the metal. Different metal ions and different porphyrin structures show selectivity for different anions. For example, the use of In (III)(25), Mn(III)(2, 8, 18), Sn(IV)(9, 20), Ru(II)(17), and Co(III)(2, 16, 17, 21) porphyrins has led to the design of a variety of anion sensors with analytically useful selectivities for chloride, thiocyanate, salicylate, thiocyanate, and nitrite/thiocyanate, respectively. However, several of the metalloporphyrin-based anion-selective electrodes exhibit much larger response slopes than the theoretical Nernstian slope.
This “super-Nernstian” behavior has been reported to originate from the dimerization of the electrode response. Therefore, metalloporphyrin-based membranes must normally be metalloporphyrins via hydroxide ion bridges. Briefly, the membranes are considered to respond to two ions, for example chloride and hydroxide, simultaneously, which, theoretically, results in a pH codependence of measured in pH-buffered samples.
The electrode slopes for the metalloporphyrin-based membranes often change with different amounts of ion exchanger used in the membranes. (3, 25, 32) The super-Nernstian response slope has been explained on the basis of the extremely small, and drastically changing, concentration of uncomplexed ionophore in the membrane. As the concentration of sample anion is increased, for example, hydroxide ions are expelled from the membrane on the basis of the ion-exchange equilibrium. This process starts breaking down the porphyrin dimers, which are held together by hydroxide bridges. (31) Consequently, the concentration of uncomplexed ionophore in the membrane starts increasing, which leads to a decrease in the uncomplexed chloride concentration in the membrane. This membrane concentration decrease upon increasing the sample concentration leads to the apparently super-Nernstian response slope.
Others have argued that the super-Nemstian slope described above indicates a higher sensor sensitivity and may therefore be beneficial. However, it was reported that the response time of such membrane electrodes is prohibitively slow. (30) Moreover, the response model demands that the membrane should be cross-responsive to another ion as well, in this case to OH− (or H+). To our knowledge, no experimental evidence has yet been presented in the literature that would support this second point, at least for the InIIIoctaethylporphyrin studied here (see FIG. 1 for structure).
The properties and applications of the well-established chloride ionophore, indiumIIIoctaethylporphyrin (In(OEP)Cl), in ion-selective electrodes and optodes have been studied extensively. This ionophore may easily form dimers via hydroxide ion bridges in polymeric membranes. The monomer-dimer equilibrium has been used for the preparation of a chloride-sensing optode. Still, the dimerization of metalloporphyrins leads to several disadvantages in ion sensors.
While ISE membranes based on metalloporphyrins should suffer from a pH cross response, the formation of indium porphyrin dimers also leads to a much reduced sensor lifetime. (36) The dimeric species is not very soluble in plasticized PVC membranes and crystallization easily occurs once the membranes or films contact an aqueous solution.
Alternative matrixes for solid-state sensors such as polyurethane (23) and silicon rubber (35) have been studied for preparing metalloporphyrin-based ion-selective membranes. Unfortunately, similar to plasticized PVC membrane, the sensors also showed super-Nernstian responses and a short lifetime. Only the In(OEP)Cl-based polyurethane membrane was reported to exhibit a Nernstian slope to chloride ion but not to other more selective anions. The formation of the dimers in these membrane matrixes was confirmed by the appearance of the Soret band at 390 nm for the dimeric species when the membranes were soaked in aqueous solution. (23) 
The covalent attachment of ionophores to a methacrylate copolymer has been studied by a number of research groups (4, 15, 22) and by our own laboratory (27). Recently, a methyl methacrylate and decyl methacrylate copolymer was developed for preparing plasticizer-free ion sensors (28) in order to overcome the disadvantages of plasticized PVC membranes, including the leaching of plasticizer in miniaturized sensing configurations. (5, 9, 33, 34) 